Wireless communication systems are widely known in which a terminal, subscriber station or user equipment (henceforth referred to as a UE for convenience) communicates wirelessly with a base station (or access point) by use of a certain radio access technology (RAT). Examples of such a RAT include the 3GPP family of standards including GSM, GPRS, UMTS and LTE, as well as WiMAX (IEEE802.16), CDMA and Wi-Fi (IEEE802.11).
Although conventionally, a UE employs only one RAT at a time for its communication, UEs such as smartphones are increasingly capable of supporting more than one RAT simultaneously and moreover, several radio access networks (RANs) employing various RATs may be available in the same place, offering the possibility of multi-RAT communication to increase the overall bandwidth available to the UE. Since each RAT available in a given area may have its own base station, this implies that the UE is able to send or receive data to and from multiple base stations (and thus via multiple cells, in the case of cellular systems such as 3GPP or WiMAX) simultaneously.
Henceforth, for convenience, the term “RAT” is also used to denote a wireless communication system employing a specific RAT. Thus, “multi-RAT communication” means communication via a plurality of wireless communication systems which involve the use of a plurality of different RATs. The term “network” is used henceforth to denote the totality of all such wireless communication systems within a given geographical area, except as demanded otherwise by the context.
Similar technologies where the base stations belong to the same RAT (Radio Access Technology), such as Carrier Aggregation (CA) or Cooperative Multi-point operation (CoMP), have been introduced into 3GPP since LTE release 10. In CA two or more Component Carriers (CCs) at different frequencies are aggregated in order to support wider transmission bandwidths up to 100 MHz. A UE may simultaneously receive or transmit on one or multiple CCs depending on its capabilities. In CoMP the cooperating base stations operate at the same carrier frequency. Details of CA and CoMP as applied to LTE are given in the 3GPP standard TS36.300, hereby incorporated by reference.
Where the base stations support different RATs, the co-operation becomes more difficult. Before discussing the problems involved, it may be helpful to outline the protocol layers involved in a wireless communication system, taking LTE as an example.
As is well known, current wireless communication systems are constructed by dividing the tasks to be performed among a plurality of layered protocols, each node or entity in the system being equipped to process data at various layers (or levels within a layer) in a protocol stack, with the protocols at corresponding layers notionally communicating with each other. Although ultimately all signalling in the system is carried by the lowest, physical layer, this hierarchical arrangement allows each layer to be considered independently.
FIG. 1 shows a protocol stack in each of three main types of nodes in an LTE-based wireless communication system. These nodes are the UE 10 (subscriber station such as a mobile handset), an eNodeB 12 (the base station in an LTE system, also called eNB), and a Mobility Management Entity or MME 16 (a higher-level node for controlling mobility of UEs, in other words handovers between eNodeBs, and for setting up “bearers” as discussed below). As shown in FIG. 1, apart from non-access stratum (NAS) protocols, all the protocols terminate in the eNodeB 12 on the network side.
The horizontal bands in the Figure represent individual protocols within the protocol stack of each node in the system, and each protocol is part of a particular protocol layer within the well-known OSI model. With respect to a given node, each protocol can be considered to reside in a functional module or “entity” which can be considered separately from protocols in other layers. This allows, among other things, for the use of the concept of “radio bearers”, which provide a kind of tunnel between peer entities in the base station and UE at a given protocol level for user data or control signalling. Radio bearers are associated with “logical channels” which link SAPs (Service Access Points) for peer-to-peer communication between the MAC and RLC protocol layers discussed below.
Packets belonging to the same radio bearer get the same end-to-end treatment in the network. There are two main bearer types, Guaranteed Bit Rate (GBR) and non-GBR. For GBR bearers, the network guarantees a certain bit rate to be available for the bearer at any time. The bearers, both GBR and non-GBR are further characterized by a Maximum Bit Rate (MBR), which limits the maximum rate that the network will provide for the given bearer. In this way it is possible for each radio bearer to provide a certain quality of service, QoS. For each radio bearer, which exists between the UE 10 and the eNodeB 12, there is a corresponding access bearer between the eNodeB and a Packet Data Network Gateway, PDN GW (not shown).
FIG. 2 is a slightly less conceptual view than FIG. 1, showing the protocol stack for one node and concentrating on the downlink (that is, the direction of transmission from the network to the UE). FIG. 2 illustrates how packets are exchanged between protocols at different layers, and shows the effect of Radio Resource Control, RRC on managing various protocols. The protocol stack in FIG. 2 is for handling user traffic (such as data being downloaded) and is referred to as the “user plane”, as distinct from the “control plane” used to carry network signalling.
As indicated in FIGS. 1 and 2, there is a physical layer protocol PHY at the lowest level, Layer 1, responsible for actual wireless transmission of data over the air, using the frequency band(s) of the RAT in use, and employing the transmission scheme of that RAT; for example, in the case of the downlink in LTE, this is orthogonal frequency division multiplexing (OFDM). In LTE, the unit of data transfer in the PHY is the Transport Block (TB). The received TBs are passed from the PHY layer to the next-higher layer (MAC) once per Transmission Time Interval (TTI) of 1 ms. Scheduling can be performed in units of 1 TTI or more, in other words on a timescale as short as 1 ms.
Thus, in the case of the downlink, radio signals at the PHY level arrive at the receiver and processed/decoded to reconstruct the transport blocks and recover data packets, which then are processed in successively-higher levels in the protocol stack. Incidentally, within each protocol the packets are referred to as “protocol data units” (PDUs) and the PDUs of one level in the stack form so-called Service Data Units (SDUs) of the next stage, possibly after concatenation or segmentation. Each TB from the PHY corresponds to a MAC PDU.
Above the PHY there are the layer-2 protocols MAC, RLC and PDCP.
MAC stands for Media Access Control and is responsible for managing the so-called hybrid ARQ function (see below), and for extracting different logical channels out of the transport block for the higher layers. Format selection and measurements provide information about the network that is needed for managing the entire network.
Logical channels exist at the top of the MAC. They represent data transfer services offered by the MAC and are defined by what type of information they carry. Types of logical channels include control channels (for control plane data) and traffic channels (for user plane data). Transport channels are in the transport blocks at the bottom of the MAC. They represent data transfer services offered by the PHY and are defined by how the information is carried, different physical layer modulations and the way they are encoded.
The Hybrid Automatic Repeat-request (HARQ) process, done in combination between the MAC and the PHY, allows retransmission of transport blocks for error recovery. The retransmission is performed by the PHY, and the MAC performs the management and signalling. The MAC indicates a NACK when there's a transport block CRC failure; the PHY usually indicates that failure. Retransmission is done by the eNodeB or the sender on the downlink using a different type of coding. The coding is sent and maintained in buffers in the eNodeB. Eventually, after one or two attempts, there will be enough data to reconstruct the transport blocks.
The MAC layer provides RLC PDUs to the next layer-2 protocol, RLC. RLC stands for Radio Link Control, and performs segmentation and reassembly and operates in three modes: transparent mode (TM), acknowledged mode (AM) and unacknowledged mode (UM). These are used by different radio bearers for different purposes. The RLC provides in-sequence delivery and duplicate detection.
Other wireless communication systems such as UMTS and WiMAX also employ RLC. Although Wi-Fi (IEEE802.11) does not employ a RLC protocol as such, the logical link control (LLC) layer in Wi-Fi has a similar role.
The next protocol in the stack above RLC, still within layer-2 of the OSI model, is PDCP. PDCP stands for Packet Data Control Protocol and, being of particular interest for present purposes, is described in some detail. Further details can be found in 3GPP standard TS 36.323, hereby also incorporated by reference.
ROHC referred to below stands for Robust Header Compression and refers to a technique used to reduce the header size of packets in LTE. Since LTE is completely Internet Protocol (IP)-based, voice calls have to be carried as IP packets using Voice over IP (VoIP) and without some measure to reduce the header size, this would be inefficient.
PDCP functions in the user plane include decryption, ROHC header decompression, sequence numbering and duplicate removal. PDCP functions in the control plane include decryption, integrity protection, sequence numbering and duplicate removal. There is one PDCP entity (in other words, PDCP instance) per radio bearer. Therefore, different PDCP entities exist which are associated with either the control plane or the user plane depending on the type of bearer.
FIG. 3, taken from the above mentioned TS36.323, is a functional view of the PDCP layer. In this Figure, u-plane denotes the user plane and c-plane, the control plane. The left-hand portion of the Figure show functional blocks involved on the uplink and the right-hand side shows the functions performed on the downlink.
As shown in FIG. 3, the PDCP layer is responsible for various tasks including:                Sequence numbering, which allows in-order delivery of packets, and duplicate detection: if the PDCP layer receives packets with the same sequence number, then it discards duplicates and does not send them to upper layers        Header compression and decompression for user plane data        Integrity protection and verification for control plane data (however, there is no integrity protection offered to the user plane data)        Ciphering and Deciphering of user plane and control plane data        Addition/removal of a PDCP Header        (not shown) Security and Handover functions.        
There is one to one correspondence between a PDCP SDU and a PDCP PDU. That is, there is no segmentation or concatenation in the PDCP layer. Addition of a PDCP header, applying compression and security on the PDCP SDU makes a PDCP PDU. Similarly deciphering, decompression and removal of the PDCP header makes a PDCP SDU from a PDCP PDU.
In LTE, the above mentioned radio bearers (RBs) are defined at various protocol levels including PDCP. There are two kinds of PDCP bearers: SRB (Signalling Radio Bearer) and DRB (Dedicated Radio Bearer). There are only two SRBs—SRB1 and SRB2. These are used by control plane protocol to send the packets to the UE. DRBs are used for sending voice and data, and as many DRBs are set up as the number of services or QoS streams required by the terminal. When a DRB is set up, a Logical Channel Identity (LCID) will be assigned to this DRB for UL and DL. In this sense, it may be said that one logical channel (LC) conventionally corresponds to one RB. For the purpose of resource allocation, the logical channels may in turn be assigned to Logical Channel Groups (LCGs). Conventionally, a given LCID or LCG can be associated with only one RAT.
Layer 3 protocols in the UE include RRC or Radio Resource Control, which is responsible for connection management, bearer control, and handovers to other base stations, UE measurement reporting, and QoS management.
Finally NAS stands for Non-Access Stratum which forms the highest-level of communication between the UE 10 and MME 16. The layers under the NAS are also referred to as the Access Stratum (AS) since they concern the radio access network which terminates at the eNodeB. NAS protocols support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a packet data network gateway, PDN GW. They define the rules for a mapping between parameters during inter-system mobility with 3G networks or non-3GPP access networks.
Returning now to the scenario of CA within LTE, the typical Layer 2 structures for downlink and uplink with CA configured in LTE networks are illustrated in FIGS. 4 and 5 respectively. As is apparent from these Figures, the multi-carrier nature of the physical layer is only exposed to the MAC layer for which one HARQ entity is required per serving cell. In both uplink and downlink, there is one independent hybrid-ARQ entity per serving cell and one transport block is generated per TTI per serving cell in the absence of spatial multiplexing. Each transport block and its potential HARQ retransmissions are mapped to a single serving cell.
When CA is configured, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information (e.g. Tracking Area Identity, TAI), and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as the Primary Cell (PCell). Generally, one carrier corresponds to one cell. In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC).
However, the above discussion relates to a single RAT (namely, LTE). The problem addressed by this invention is in a wireless communication system where multiple radio access technologies (e.g. GSM, UMTS, LTE and beyond, WiMAX, WiFi, etc.) are available in the whole network or in certain areas, such as city centres (either full time, or during peak hours only).
For simplicity, LTE and WiFi will be used as an example of multiple RATs co-existing in a wireless communication system. FIGS. 6(A) and 6(B) illustrate two examples of typical deployment scenarios in such system; in case (A), the LTE eNB and WiFi AP are separated (in other words provided by different pieces of equipment), while in case (B) the LTE eNB and WiFi AP are co-located, in other words a single unit acts as a combined LTE base station and Wi-Fi access point. In both cases, the UEs are assumed to be dual (or more) mode devices having a WiFi interface. It is further assumed that there is some form of backhaul network (such as broadband Internet) connecting both the eNB and AP to a core network.
Based on the current 3GPP standard as set out in TS36.300, the eNB in an LTE system is responsible for managing resource scheduling for both uplink and downlink channels. In order to utilise the shared channel resources (by multiple UEs) efficiently, a scheduling function is used in the MAC layer. The MAC entity in the eNB includes dynamic resource schedulers that allocate physical layer resources for the DL-SCH and UL-SCH transport channels. Different schedulers operate for the DL-SCH and UL-SCH. The scheduler should take account of the traffic volume and the QoS requirements of each UE and associated radio bearers, when sharing resources between UEs. Schedulers may assign resources taking account the radio conditions at the UE identified through measurements made at the eNB and/or reported by the UE. Radio resource allocations can be valid for one or multiple TTIs.
When CA is configured, a UE may be scheduled over multiple serving cells simultaneously. However, in multi-RAT scenarios, resource scheduling becomes very challenging; especially for the case of FIG. 6(A) where LTE eNB and WiFi AP are separated, and backhaul support cannot be assumed to be ideal for the information exchange between the multi-RAT nodes. The key issue in such scenarios is how to co-ordinate the radio resource scheduling in multiple nodes of different RATs in order to achieve efficient multi-flow aggregation. This is an issue in both UL and DL; however the present invention is mainly concerned with the downlink.